1. Field of the Invention
The present invention relates to improvements in or relating to electrical insulators, in particular, high voltage electrical insulators of the type having a central portion from which one or more sheds extend outwardly.
2. Description of Related Art
High voltage electrical insulators of the type suitable for use in power supply systems generally comprise a central stem with sheds extending outwardly therefrom at spaced intervals. Such insulators are typically made of porcelain although other insulating materials may be used.
The main problems encountered with high voltage (h.v.) insulators when used outdoors are those of pollution and wetting of the porcelain. With HVDC wall bushings, in particular near-horizontal bushings, flashover problems arise not only when the bushing is polluted but also when it is clean due to non-uniform wetting when the zone near the earth is well protected from rain by the wall of the valve hall through which it projects.
When there is heavy pollution near-vertical a.c insulators may be washed frequently by means of high pressure water Jets or sprays to remove the pollution layer from the surfaces of the insulator. During the washing process there arises the risk of flashover occurring because water with a high pollution content will be running over the insulator. Accordingly, it has long been considered desirable to reduce the risk of the occurrence of flashover in a variety of heavy wetting conditions. This can be achieved by increasing the flashover voltage or strength of an insulator such that at working voltages flashover will not occur.
It is already known that the provision of several dielectric barriers known as "booster sheds" placed close to the upper surface of a shed on a near-vertical insulator will reduce the risk of flashover. With HVDC wall bushings in the near-horizontal position booster sheds have also been effective in increasing flashover voltage. This is achieved because the flashover strength of the local insulator sheds where the booster sheds are situated is increased. Thus, in critical conditions the voltage on the porcelain sheds near the booster sheds may be higher than on the remaining porcelain sheds. This may have resulted for d.c bushings in the puncturing both of the porcelain sheds as well as the booster sheds during testing. GB1542845 describes such a booster shed which comprises a sheet of dielectric material in the form of a truncated cone which is arranged such that it lies close to the upper surface of a shed. The booster shed is spaced from the surface (or face) of the shed with the lesser angle to the insulator axis by means of small projections which extend downwardly from the lower surface of the booster shed. In addition, the inner edge of the booster shed, which lies closest to the central stem of the insulator, is spaced from the central stem by means of tongues so that there is as little contact between the booster shed and the insulator as possible. The booster shed itself is in the form of a truncated cone formed by removing a sector from an annulus of dielectric material and joining the edges securely by locking means, such as tight-fitting pegs in holes with an insulating filler in the joint to exclude moisture.
Although the booster sheds disclosed in GB1542845 were found to be effective against the risk of flashover, the joint in the dielectric cone is susceptible to puncture, tracking or carbonisation. In addition, these booster sheds have proved expensive to manufacture because they require an elaborate moulding operation to create the various projections. The method of joining the edges of the booster shed once placed over a shed of the insulator has also proved difficult especially when the adjacent sheds of a porcelain insulator are closely spaced. By using a greater number of barriers than the number of booster sheds previously used it is not necessary to have each barrier capable of withstanding the same voltage as the booster sheds had to. In addition, for a given service voltage, a greater number of barriers will reduce the maximum voltage across individual porcelain sheds. The voltage will increase with parameters such as the proximity of the barrier to the porcelain shed, the radial length of the barrier adjacent to the porcelain shed and the overhang of the barrier beyond the porcelain shed. It is therefore a matter of balancing the number of barriers against these parameters, the risk of damage to the porcelain sheds, and the risk of damage to the life of the barriers to obtain the necessary increase in flashover voltage so that in local conditions, for any kind of pollution, there will be few or no flashovers at the working voltage.
There is also the risk of internal puncture of the insulator which is mainly relevant to HVDC bushings and is a serious practical problem. Internal punctures can result in the loss of oil from within the insulator which could lead to fire and hence the loss of a converter station for a substantial period of time. FIG. 1A shows a graph which depicts how the voltage varies along a non-uniformly wetted HVDC wall bushing which is at an angle of approximately 15.degree. to the horizontal between the external high-voltage end and the earthed end which is located at the surface of the wall plotted along the axis of the bushing. Line A shows an idealised representation of the voltage along the surface of the internal paper core of the bushing. Curve B shows the idealised voltage distribution on the external surface of the bushing with a dry zone established at the wall end. The high resistance of this dry zone creates a high voltage drop across it and a corresponding high radial voltage difference indicated by the difference between lines A and B at the edge of the dry zone. If additional dry zones are produced by barriers this will reduce the maximum radial stress at the expense of higher stresses across the sheds along the insulator surface. Curve C depicts the variation in external voltage with four barriers in positions W, X, Y and Z on the insulator. Clearly, the greater the number of barriers the more line C will approximate to the idealised line A. However, it should be pointed out that the graph in FIG. 1A is only a simplified representation of the voltage variation.
Since the mechanism of the flashover process is different for non-uniformly wetted horizontal d.c bushings than for a normal vertical a.c insulator with uniform wetting, different design criteria must be considered. The barriers function differently for direct and alternating voltages and on vertical and near horizontal insulators the following are important considerations: